There is increasing interest in performing many key signal processing functions in the optical domain. Optical correlation, or matched filtering, of digital signals is one such key function that is used to compare a given bit pattern with one that is ‘stored’ in an optical correlator. The current and potential applications of digital optical correlators are introduced and the process by which many optical correlators operate is explained in detail. In addition, a number of specific optical correlator designs are presented and explored in greater depth. Significant limitations and implementation challenges as well as potential solutions are also discussed.